FORMATION OF GERMINAL LAYERS IN OHELONIA. 25 



5. At the posterior wall (i.e. floor) of the blastoporic pas- 

 sage the ectoblast and the entoblast are fused, and from the 

 point of fusion the mesoblast is being given ofl" posteriorly in 

 all directions for the space of 180°. 



6. Also, behind the place where the two primary layers are 

 thus fused, the ectoblast is giving off cells to the mesoblast 

 along the median line (fig. 8). This is the line of the primi- 

 tive streak. It is very short and is present in only two or 

 three sections. 



7. The mesoblastic mass derived from the two sources men- 

 tioned in (5) and (6) is unpaired and constitutes the transverse 

 swelling in the posterior part of the embryonic shield, " the 

 sickle." This is the only place where the mesoblast is present 

 at this stage. 



8. The median mass formed by the fusion of the three 

 layers at the posterior wall of the blastoporic passage appears 

 for a short space on the dorsal surface (fig. 9) — the remnant of 

 the yolk-plug of Rusconi. 



Formation of the Mesoblast and of the Chorda 

 Dorsalis. 



In the previous stage, the mesoblast was found only in the 

 region behind the blastopore. We may now proceed to 

 describe its formation in front of the blastopore. We call atten- 

 tion first to the embryo represented in figs. 2 a and b. It was 

 taken out exactly forty-eight hours after the deposition, but as 

 the weather was unusually cold for the season during the 

 interval, it has made very little progress in development, and 

 is not as far advanced as many thirty-six hours old. As before, 

 a dorsal and a ventral view of the embryonic shield is given, 

 although these are not taken in this case from the same 

 embryo. The shape of the shield has not changed materially 

 from the previous stage. In the dorsal view (fig. 2 a) the 

 blastopore has assumed a horseshoe shape, and is more of a 

 slit than before. Occupying the concavity of the horseshoe 

 is the rudimentary yolk-plug. Round the blastopore, and 



